I have some problems with my powerline LAN: The network works perfectly fine 80% of the time, but 20% of the time it is really slow. E.g. if PC1, PC2 and PC3 are playing an online game the ping drops from 50 ms to 200 ms once or twice per hour. These performance spikes only last a few minutes and after that, everything goes back to normal again.

I recently changed the adapter on PC1 and the frequency of lag issues decreased

Sometimes PC3 can't connect to the network at all

Adapter4 is not connected to the UPS

PC5 is used a server and does backups for all the other computers

What could cause this issues?

The UPS that was recently added?

The increased number of devices connected to the power outlet on PC4?

A broken network card?

A broken powerline LAN adapter?

I can do some tests with the network, but the amount of the time where PC4, PC5 or the UPS can be disconnected from the network or switched off is very limited.

1: I am not really sure about this one. I don't play online games so frequently that I can pinpoint it to that exact date.

Answers to comments on this question:

My bet is that it's just network load. It could be two PCs talking to
each other, saturating the powerline. It could be one PC saturating
the Internet link

I can somehow exclude the powerline saturation issue, because I had a case where only PC1 and PC4 (PC4 is not connected to the powerline network) were online and the lag issue was still there.

Regarding the Internet link saturation: What I think is odd is that it happens periodically and only for a short time. I would understand a relation between heavy Internet usage (downloads, streaming, etc.) and performance problems for the other clients, but the problems would be long and random in this case.

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Update:

Did some testing yesterday: When PC4 is online, the lag issues start. Normal web browsing on PC4 causes periodic performance issues on all other clients.

Please tell me why you think it is the network card's fault. Is this a common behavior of a broken network card? I'm quite sure all the computers in use have a built-in network card, so I will most likely have to buy an extra one to test this theory.
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xslApr 23 '12 at 20:48

I have had cases where a faulty card bombarded the network with packages and caused collisions.
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harrymcApr 24 '12 at 4:53

Thank you! Is it also common that these issues happen only once or twice per hour?
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xslApr 24 '12 at 6:46

No, in the cases I had this problem was continuous. Of course one can never know with hardware, but try to boot PC4 in Safe mode with Network. If it doesn't happen anymore, then this might be some installed application that is overloading the network. PC4 might even possibly be infected. For the UPS issue : Does this happen when some computers are turned off, and have you checked the power requirements.
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harrymcApr 24 '12 at 6:53

Thanks a lot. I will try the safe mode. The UPS works fine. I thought the UPS might be the problem, because it could be in conflict with the power-line network. When PC4 and PC5 are turned off, the lag issues disappear.
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xslApr 24 '12 at 7:27

Now that you have narrowed it down to one PC (sortof), two iterations of "bisectination" are complete. (I say sortof, because it could just be PC4's location or external components, not the PC itself)

dump the UPS on that box (just because it is fast and easy) - fixed? - remove/replace UPS

use a different PC in PC4's place using all of its cables (same lan adapter, UPS, cables etc... that PC4 was using) - fixed? - must be in PC4 (lan card or its driver or internal interference from some other component)

if replacing PC4 did not fix it, it could be lan adapter or "interconnecting wiring" - try using a different lan adapter and/or swapping out your lan cables (bad ethernet cables are a fairly common problem and could have been damaged while you were shifting stuff around to put in your shiny new UPS)

if none of that works try a different outlet (a long extension cord will work temporarily), something on that circuit could be causing interference

Ethernet over Powerline is notoriously sensitive to noise in the power line that doesn't bother any power devices. One place I lived, every time the refrigerator compressor turned on to cool the refrigerator the EoP pretty much stopped working. Now in that case it was near its limits to begin with, but it wouldn't surprise me if you had something similar. Any electric motor (refrigerator, dishwasher, clothes washer, blower for central heat, etc.) can cause serious degradation of the network and there's not much you can do about it.

What kind of UPS do you have? Where do you have power filtering (e.g. surge suppression)? One thing to note is that if a surge suppressor is plugged into the same outlet (or even the same circuit but a different outlet nearby) as the EoP adapter it can degrade network performance. So you may have lowered your safety margin by recently plugging in a new surge supressor and now suffering from refrigerator noise where you didn't used to.

I'd start by making sure that at the very least you don't have the UPS or any surge protectors plugged in within 6 feet of an EoP adapter.

Thank you for your comment. If PC4 is not switched on, but the UPS is, everything is working fine. If PC4 is online, the lag issues appear. The UPS is indeed within 6 feets of the EoP adapter and I most likely can't change anything about that. Do you think the additional device on the UPS (PC4) could be causing the problem?
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xslApr 30 '12 at 8:55

@xsl, Yes, depending on the type of UPS, additional load can cause additional noise on the line, which could cause EoP interference. What happens to the EoP when you unplug the UPS with PC4 turned on? I cannot tell from your posts what the electrical configuration is with respect to Adapter4, PC4, the UPS, and PC5. Is PC4 plugged into the UPS? Can you move Adapter4 or the UPS to a different electrical circuit? How many different circuits are the 4 adapters on? Are they all on the same electrical phase? Best if all the EoP are on the same phase and the UPS is on a different phase.
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Old ProApr 30 '12 at 15:47